This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-186330, filed Jun. 30, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a serial signal transmission apparatus for realizing a high-speed serial interface.
Recently, various portable, battery-driven, notebook type personal computers (to be referred to as notebook PCs hereinafter) have been developed. Some notebook PCs are designed to be attached to expansion units to expand the functions as needed. To allow a notebook PC body to effectively use the resources of an expansion unit, it is important to connect the bus of the notebook PC body to the bus in the expansion unit. With this bus connection, devices on the bus in the expansion unit can be handled in the same manner as devices in the notebook PC body.
In many personal computers, PCI (Peripheral Component Interface) buses are used. Bus connection between a notebook PC body and an expansion unit is generally performed by physically connecting PCI buses on the notebook PC body side and expansion unit side through docking connectors, each having many pins corresponding to the number of signal lines of each PCI bus, prepared on the notebook PC body side and expansion unit side.
According to this arrangement, however, a large area is required to mount a docking connector. This causes a disadvantage in attaining reductions in the size and profile of the notebook PC body. In addition, the connector mounting position on the notebook PC body side needs to match that on the expansion unit side. This imposes limitations on a physical housing structure in production development.
Under the circumstances, demands have arisen for development of a technique for transferring transactions between. PCI buses through a high-speed serial interface. With the use of a high-speed serial interface, a notebook PC body can be connected to an expansion unit through a thin, flexible serial cable.
As schemes of transmitting high-speed serial signals, two modes, i.e., a differential mode and a bipolar transfer mode, have attracted attention.
The differential mode is a scheme using a pair of signals whose phases are inverted from each other. In this scheme, binary serial data of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d are transmitted as a phase-inverted signal pair.
The bipolar transfer mode is a scheme of transmitting binary serial data of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d at three voltage levels, i.e., +V, 0, and xe2x88x92V. When data changes, the sign is inverted. When identical data is to be transmitted, a voltage level of 0 is output. In the bipolar transfer mode, if data transfer in the long term, the numbers of +V and xe2x88x92V signals become almost equal.
When a high-speed signal which is low in ability to recover and likely to cause an operation error in a case of data loss due to noise is to be transferred, a transformer as an antistatic means is preferably inserted in signal lines to separate DC components. In signal transfer using a transformer, the following two problems are posed, and hence care must be taken for design.
1) To prevent a transformer from being saturated, a current must be prevented from continuously flowing in one direction.
2) The total time in which a voltage is applied to the transformer in one direction is made almost equal to the total time in which a voltage is applied to the transformer in the other direction over a long-term span. If the total time in which a voltage is generated in one direction is more than that in the other direction, voltage waveforms generated on the secondary side shift in voltage level as a whole, although the voltage amplitude remains unchanged.
As a method of solving these two problems, transfer based on the bipolar transfer mode is an optimal scheme. In developing an LSI using this mode, a long development time is required because there is no library for LSIs only requiring a short delivery/development time, e.g., gate arrays and standard cells.
In the case of the differential mode, a library for LSIs such as gate arrays and standard cells has been prepared as standard, only a short development time is required. It is, however, difficult to solve the problems 1) and 2). As a countermeasure against these problems, the nBmB scheme, which increases the data length and encodes it for transmission is available. However, extra data for coding is added to the data, the transfer rate of the target data decreases.
The present invention has been made in consideration of the above situation, and has as its object to provide a serial signal transmission apparatus which can realize serial transmission suitable for signal lines in which a transformer is inserted, by using existing differential buffers, and can perform serial transfer with sufficiently high reliability without decreasing a transfer rate.
In order to achieve the above object, according to the present invention, there is provided a serial signal transmission apparatus for transmitting serial data through a signal line pair in which a transformer is inserted. The serial signal transmission apparatus comprises a first differential output buffer whose positive and negative output terminals are respectively connected to two lines of the signal line pair, and a second differential output buffer whose positive and negative output terminals are respectively connected to the two lines of the signal line pair.
The signal line pair is driven by using the first and second differential output buffers to convert each binary data constituting serial data into ternary data including a first state in which a current flows in the transformer in a positive direction, a second state in which a current flows in the transformer in a negative direction, and a third state in which no current flows in the transformer, and the ternary data is output onto the signal line pair.
In this serial signal transmission apparatus, two differential output buffers are connected in parallel with each other, and a signal line pair is driven by these two differential output buffers. In this case, a voltage between the signal lines of the signal line pair is determined by a combination of outputs from the two differential output buffers. Ternary data can therefore be transmitted by using, for example, a state in which both the two differential output buffers output currents in the positive direction as the first state in which a current flows in the transformer in the positive direction, a state in which both the buffers output currents in the negative direction as the second state in which a current flows in the transformer in the negative direction, and a state in which one differential output buffer outputs a current in the positive direction, and the other differential output buffer outputs a current in the negative direction so that the output currents cancel out each other as the third state in which no current flows in the transformer. By using this ternary data, signal transmission based on the abovementioned bipolar transfer mode can be realized. This makes it possible to solve problems, e.g., magnetic saturation of the transformer and a shift in the offset level of a signal on the secondary side of the transformer.
On the receiving side as well, by connecting differential input buffers in parallel with each other, ternary data transferred by the bipolar transfer mode can be easily detected. In this case, it is preferable that the apparatus further comprise means for applying a predetermined bias voltage between the reference signal input terminals of the first and second differential input buffers and the signal line pair on the secondary side of the transformer so as to produce potential differences between the reference signal input terminals and the signal line pair on the secondary side of the transformer in the third state. This makes it possible to correctly detect the third state.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.